This application claims Paris Convention priority of DE 101 50 131.5 filed Oct. 11, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns an NMR (nuclear magnetic resonance) resonator device with at least one RF (radio frequency) resonator for emitting and/or receiving RF signals at one or more desired resonance frequencies to and/or from a measuring sample in an investigational volume, disposed about a coordinate origin (x,y,z=0), of an NMR apparatus with a means for producing a homogeneous magnetic field B0 in the direction of a z axis, wherein superconducting conductor structures of the RF resonator, which act inductively and partially also capacitively, are disposed between z=xe2x88x92|z1| and z=+|z2| substantially on a surface which is translation-invariant (=z-invariant) in the z direction at a radial (x,y) separation from the measuring sample.
An arrangement of this type is known from U.S. Pat. No. 5,585,723.
The present invention concerns the field of high-resolution nuclear magnetic resonance (NMR), in particular a configuration of superconducting resonators for receiving the NMR signal from the NMR measuring sample.
Although NMR is a very useful method for structure analysis of chemical compounds, it is not very sensitive. To increase the sensitivity, according to current prior art, cooled normally conducting and in particular superconducting resonators are used which considerably increase the S/N ratio. [1] describes e.g. such resonators.
The main problem with the use of superconductors for the production of NMR resonators is their static magnetization. In a superconductor of type II, this magnetization is produced by induced currents which flow in closed paths within the superconductor and which depend on the history of the superconductor [2]. These currents can produce strong disturbances in the homogeneous field of the active region of the measuring sample which cause distortion of the resonance lines in the NMR spectrum. As long as the external conditions remain unchanged, these currents continue to flow for a nearly unlimited length of time due to the zero resistance of the superconductor.
Methods ([2], [3], [4]) for minimizing this magnetization have been published. They are all complicated and have further disadvantages which are described below. Superconducting coil arrangements have also been described [5] which minimize their disturbing fields through limitation of the active measuring region using normally conducting connecting elements. These coil arrangements [5] are superior to the above-mentioned solutions of methods [1-3] with respect to fill factor and attainable RF field strengths, but have the disadvantage that the Q-value of the RF resonator is significantly reduced by the normally conducting components which must be used.
The known measures for minimizing the influence of magnetization are:
1. Division of the width of the superconductor into n individual longitudinal strips ([1], [5]), wherein n should be as large as possible. This reduces the maximum currents which can flow in these longitudinal strips and therefore also the maximum possible magnetization of the superconductor by the factor n.
2. Prevent magnetization completely by first cooling the superconductor when it is positioned in the static field of the magnet. Patent [4] additionally recommends slow cooling.
3. Largely eliminate existing magnetization by means of a demagnetization process ([2], [3]). This is achieved by a sequence of decreasing transverse magnetic fields which act on the superconductor. A current structure with opposite current regions is thereby induced in the superconductor, with the sum of the individual magnetic field contributions cancelling to a good approximation.
These known methods have significant disadvantages:
1. Although the measure described in 1 considerably reduces magnetization, the remaining residual magnetization is generally still too high for adequate performance in high-resolution NMR applications.
2. The methods described in 2 and 3 can function satisfactorily only when the position of the superconductor with respect to the static magnetic field remains exactly the same during the entire measuring process and when the static magnetic field itself also remains unchanged during this period. The principal problem is the constancy of the angular position which is usually insufficient in practice. A tilting of merely 0.1 degrees relative to the static magnetic field can produce shielding currents in the superconductor of such a strength that the field homogeneity is deteriorated to an unacceptable degree.
Tilting causes an additional transverse magnetic flux to be directed from the static magnetic field onto the surface of the superconductor, and since the superconductor tries to maintain the previously existing flux, it counteracts with additional surface currents such that the total flux through the superconductor once more corresponds to the original value. These surface currents produce an inhomogeneous field at the location of the measuring sample thereby unduly deteriorating the required spectral resolution.
3. The methods ([2], [3], [4]) are difficult to carry out and require additional devices in the critical region of the NMR resonator.
In contrast thereto, it is the object of the present invention to present a new type of superconducting NMR resonator having additional superconducting conductor structures which are optimally decoupled from the actual RF resonator and which optimally compensate for the disturbing influence produced by magnetization of the superconductor.
This objective is achieved in accordance with the invention in an NMR resonator having the above-described features in that an additional compensation arrangement is provided on the z-invariant surface, which extends to values of at least z less than xe2x88x92|z1|xe2x88x920.5|r| and z greater than +|z2|+0.5|r|, wherein |r| is the minimum separation between the measuring sample and the compensation arrangement, with the compensation arrangement comprising further superconducting conductor structures which are largely RF-decoupled from the RF resonator, wherein the conductor structures of the compensation arrangement and of the RF resonator are composed of individual surface sections (xe2x80x9cZ-structuresxe2x80x9d) which comprise superconducting structures and which are disposed in the z-invariant surface to each extend across the entire length in the z direction of the conductor structures of the compensation arrangement and of the RF resonator and whose superconducting structures are disposed such that, with suitable conceptual decomposition of the surface of the Z structures into a plurality of small equally sized surface elements and with conceptual application of a homogeneous test magnetic field along the surface normal of each surface element, a magnetic dipole moment of essentially the same strength would be induced in all surface elements which differ only with respect to their z position.
The inventive resonators have many advantages:
the complicated methods ([2], [3], [4]) for demagnetisation of the superconductor are no longer required.
external disturbing fields and mechanical motion of the resonator relative to the static magnetic field produce a magnetization in the superconductor which has only minimal influence on the homogeneity of the static magnetic field in the active region of the measuring sample due to the above-mentioned compensation. This is also true when the superconductor is highly magnetized.
known coil structures [5] can be designed much more freely and therefore more effectively.
In the inventive resonator, the individual superconducting conductor parts which carry the radio frequency (RF) current and which therefore form the RF resonator are supplemented by additional superconducting conductor parts which are disposed quasi continuously within the RF resonator, which extend beyond same in the z direction and which are, to the extent possible, RF-decoupled from the RF resonator such that they do not carry RF current. FIG. 12b shows an arrangement built according to this principle, wherein the RF resonator is shown with hatched lines and the additional conductor parts, which merely serve to homogenize the B0 field in the active measuring region, are shown in black. Clearly, the superconducting material is distributed fairly homogeneously across the entire RF resonator surface and the additional conductor parts which generally leads to a uniform distribution of the dipole moments over the entire surface. This uniform distribution of the dipole moments which are produced by currents induced in the superconductor, produces a nearly homogeneous disturbing field in the measuring volume and can therefore no longer have a significant influence on the NMR spectrum.
The entire resonator is preferably assembled from conductor sections of minimum width (FIG. 17a) i.e. the minimum width possible using the lithographic production method, wherein these conductor sections can also be disposed in bundles (FIGS. 17b, 16c, 16d).
The magnetization currents in the superconductor and the field disturbances produced by them are thereby minimized in the active measuring region.
Conductor sections which are horizontally oriented, not bundled and disposed in the z direction with periodic continuity (FIG. 17a) have a higher periodicity in the z direction than they would have were they bundled (FIG. 17b). In the former case, magnetization of the superconductor therefore produces a field disturbance in the active measuring region which is smaller than in the latter case (see also FIG. 8). Bundling of the individual conductor sections can still be advantageous for constructive reasons, in particular for generating the capacitive portion of the RF resonator.
The terms transverse field, longitudinal field, NMR resonator, RF receiver coil arrangement, and RF resonator will be mentioned several times in the following description and are therefore defined in detail below:
a transverse field is a field which is oriented perpendicular to the static magnetic field B0, a longitudinal field is parallel to B0.
an NMR resonator refers to the entire resonator arrangement. It is composed of one or more, preferably 2 or 4 RF receiver coil arrangements which are disposed around the measuring volume and which are RF-coupled to one another. The RF receiver coil arrangement itself comprises the RF resonator and the compensation arrangement, wherein the RF resonator substantially represents that part of the RF receiver coil arrangement which carries the RF current.
Different embodiments will be defined below taking into consideration the above definitions.
In one particularly preferred embodiment of the inventive RF receiver coil arrangement, all surface elements which differ only with respect to their z position, contain substantially the same amount of superconducting material. This facilitates design of the surface elements such that the desired RF and magnetic properties are approximately obtained in a simple manner.
Smaller constituent elements of the superconducting structure permit a finer subdivision of the overall surface of the RF receiver coil arrangement into identical, small surface elements having as equal dipole moments as possible. The smallest dimension of these surface elements must not be less than the smallest dimension of the structural elements, since individual surface elements could otherwise fail to contain any superconductor at all thereby violating the condition of identical dipole moments per surface element. An as fine as possible surface division is required to minimize the waviness of the disturbing field in the active measuring region produced by the magnetic dipole moments of the superconducting material in the individual surface elements. Reasonably fine division can be obtained when the total number of the surface elements is larger than 50, preferably larger than 200.
The most important aspect of this divisioning is the number of identical surface elements which differ only with regard to their z position, i.e. disposed on strips oriented parallel to the z axis. This number should be larger than 20 and preferably larger than 50.
In one particularly preferred embodiment of the inventive RF receiver coil arrangement, the superconducting conductor structures of the compensation arrangement project past both sides of the RF resonator by at least half, preferably approximately twice, the extension of the RF resonator in the z direction. The edge regions of the compensation arrangement which are mainly responsible for the disturbing influences in the active measuring region are thereby spatially moved as far from the active measuring region as possible to preclude significant influence at that location.
NMR resonators are usually composed of several RF receiver coil arrangements in order to optimise the fill factor. In an advantageous embodiment of the invention, the NMR resonator therefore contains several coupled RF receiver coil arrangements, preferably 2 or 4 on different partial regions of the z-invariant surface.
In one embodiment of the invention which is particularly simple with respect to geometry and production, the superconducting conductor structures of the RF resonator(s), which act inductively and partially also capacitively, and the superconducting conductor structures of the compensation arrangement(s) are both disposed on flat substrate elements which are oriented parallel to each other and to the z axis.
In one embodiment of the inventive RF receiver coil arrangement which is particularly simple to produce, the superconducting conductor structures of the RF resonator, which act inductively and partially also capacitively, and the superconducting conductor structures of the associated compensation arrangement are disposed in the same plane. This and the previous embodiment function optimally when the working point remains in the linear region of the magnetization curve, i.e. when the magnetization of the superconductor is not too large. Specially selected e.g. zig-zag shaped conductor structures also permit optimum function outside of the linear region.
An alternative embodiment is characterized in that the superconducting structures of the RF resonator and of the associated compensation arrangement are disposed in two or more flat partial surfaces which are arranged parallel to each other, wherein the superconducting conductor structures of the RF resonator, which act inductively and partially also capacitively, and parts of the associated superconducting conductor structures of the compensation arrangement are disposed in the first partial surface and the remaining parts of the superconducting conductor structures of the compensation arrangement are disposed in the further partial surfaces. This somewhat complicated arrangement produces a further physical degree of freedom which permits very precise compensation of the disturbing influences caused by the magnetism of the superconductor, wherein optimum results can also be achieved when the working point is outside of the linear region of the magnetization curve.
In a further development of this embodiment, the separations between the partial surfaces, measured in a direction perpendicular to the partial surfaces are not more than 600 xcexcm, preferably between 50 and 200 xcexcm. These separations must be kept as small as possible to minimize magnetism compensation errors.
In an embodiment of the inventive RF receiver coil arrangement allowing fill factors which are even higher than in the flat arrangement, the superconducting conductor structures of the RF resonator(s), which act inductively and partially also capacitively, and the superconducting conductor structures of the compensation arrangement(s) are disposed on flat substrate elements which are cylindrically curved about the z direction. This, however, makes production much more difficult.
One simple embodiment of the invention is particularly preferred wherein the conductor structures of the compensation arrangement and of the RF resonator are disposed on the same partial region of the z-invariant surface. A plurality of further embodiments, which are characterized by their simple construction, can be derived from this embodiment.
In a further particularly preferred very simple embodiment of the invention, at least part of the superconducting sections of the conductor structures is disposed like strips which are either parallel or perpendicular to the z axis. This embodiment has a simple geometrical shape and serves as a starting point for the production of different z structures.
In one embodiment which increases variation options, the superconducting structures on the z-invariant surfaces have different geometric shapes, e.g. strips of different orientation and width and squares, circles, trapezoids etc. This increases the flexibility for design of the overall geometry to further optimise error compensation.
A very useful Z structure which can be used individually or multiply in an inventive RF receiver coil arrangement consists of identical, narrow superconducting structural elements which are formed of one individual conductor or of conductors disposed in groups which are disposed at close intervals, periodically and continuously in the z direction, wherein the separation between neighboring structural elements is small compared to the separation from the center of the investigational volume.
In a further very useful Z structure which can be applied individually or multiply in an inventive RF receiver coil arrangement, structural elements having one individual conductor or conductors disposed in groups have conductor sections which are oriented parallel to the z axis and which have identical or different mutual separations.
An alternative embodiment of the Z structure which can be used individually or multiply in an inventive RF receiver coil arrangement is characterized in that it is divided into several, preferably rectangular partial regions whose width equals the width of the Z structure and that these partial regions are filled by straight superconducting strips which are inclined at a desired angle with respect to the z axis, wherein this angle is either positive or negative within a partial region, and the inclined strips are disposed within a partial region at close intervals, periodically, and continuously in the z direction, wherein the separation between neighboring strips is small compared to the separation from the center of the volume under investigation, and the inclined strips continuously merge into one another at the border lines between the partial regions. This Z structure has a more complicated construction but is advantageous in that it lies on one single surface and offers perfect compensation even when the superconductor is magnetized to such an extent that its working point is outside of the linear region of the magnetization curve.
In one embodiment of the inventive RF receiver coil arrangement, the superconducting conductor structures which do not belong to the RF resonator contain a number of narrow interruptions which are distributed over parts or over the entire length of the individual superconducting conductor sections. This considerably improves RF decoupling between the RF resonator and the compensation arrangement since the eddy currents which are usually produced in the conductor structures are minimized or suppressed by the interruptions.
In a further embodiment of the invention, the RF resonator is produced from the desired regions of the Z structures in that the superconductor or parts thereof is/are rotated or displaced within small surface elements of the Z structures, and/or narrow separations between the conductors are connected in a superconducting fashion and/or narrow interruptions are introduced in the conductors and/or inclined and zig-zag shaped Z structures are used and/or the Z structures are distributed over two or more partial surfaces.
A very concrete, particularly preferred embodiment which has a relatively simple geometrical structure is characterized in that the RF receiver coil arrangement is formed of three Z structures, two with vertical strips on the left and on the right and one with horizontal straight strips in the middle.
Other possible geometric arrangements result from embodiments wherein the RF receiver coil arrangement is formed of three Z structures, two with inclined partially zig-zag shaped transverse strips on the left and on the right and an intermediate structure with transverse strips disposed periodically in the z direction. The construction of this embodiment is more complicated but is advantageous in that it lies on one single surface and provides perfect compensation even when the magnetization of the superconductor is sufficiently strong to drive its working point outside of the linear region of the magnetization curve.
In another embodiment of the invention which provides very exact compensation and many variation possibilities, the entire RF receiver coil arrangement is disposed in two planes and is composed of three Z structures, i.e. of a first and second structure with strips arranged parallel to the z axis at a defined mutual separation and a third structure with strips disposed periodically in z and transverse to the z axis, wherein the third Z structure is positioned above the first two structures and precisely covers these, wherein the RF resonator and the compensation structure are generated in that the three Z structures are partially decomposed and distributed on two separate partial surfaces which belong to one or two substrates, wherein the first partial surface contains the entire RF resonator with the main part of the compensation structure and the second partial surface contains the remaining part of the compensation structure, and the structures produced in this fashion are positioned on top of each other on the two partial surfaces such that the sum of their structures is once more equal to the sum of the original three Z structures thereby producing an operative NMR resonator.
In further preferred embodiments of the invention, at least parts of the superconducting conductor structures can contain high temperature superconducting (HTS) material. Conductors of HTS material produced in the epitaxial method have very high critical current densities with very high field strengths in the temperature range of approximately 4.2 K to 30 K, which is generally used for superconducting NMR resonators, and are therefore highly suitable for producing NMR resonators.
Finally, one embodiment of the invention is also advantageous wherein the superconducting conductor structures are formed from thin layers, preferably of a thickness of between 0.1 and 1 xcexcm which is within the scope of conventional production technology.
Further advantages can be extracted from the drawing and the description. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail by means of embodiments.